I realized early that Heroes 6 is the weakest installment of the series. It has an utterly generic story that has nothing to do with the events of the previous titles and a gameplay that lacks any inspiration. But I could have ignored that, since, despite being not a very good title, the game still has some solid entertainment.
What really killed it for me is the "always online" issue, even if you really just want to play a single-player game and don't even care about multiplayer. While being always online in Steam is usually a non-issue, having to be always online on the notoriously unstable Uplay network can be a pain.
Also if Uplay servers are down for any reason and you try to work around that by starting the client in offline-mode the game will begin giving you trouble. For example it will refuse to let you load any of your savegames, because games you played while you were online are not available when you are offline. And if it has a particularly bad day it will deactivate completely, tell you that your version of Heroes 6 is "unregistered" and demand that you go online to register your already registered game another time before you are allowed to play the game again. So that's my real issue.
However, if you need another reason, then here is a word for those of you who just happen to be die-hard fans of the series like me: please take my warning. This game does NOT continue the events of Heroes 5 and/or Dark Messiah of Might and Magic. It is a prequel that surrenders the previous story-line completely.
I mean: Bloody hell! It's not like Ubisoft left us with a cliffhanger after Heroes 5, where the dark messiah, possessed by a half-naked, smoking-hot succubus, stopped the necromancers and their alliance of the willing from dropping weapons of mass-destruction on the underworld, destroyed the skull of the seventh dragon to free the demon-lord and open the gates of hell. So that the dark lord would rise from the abyss speaking the magic words: "I am your father". You know - introduce some global warming: the lucifer-way. And right before we found out if the world was going to end and if beelzebub actually can be a picture-book daddy... curtains dropping! The end.
And here comes Heroes 6 and it starts with the words "2000 years earlier"! Wait a minute, Ubisoft: I thought we just had some serious armageddon and succubus-action going on here! What happened to good old: smash skull first, eat brains later?
So honestly: unless you are really desperate for another Might and Magic title, just skip Heroes 6 and wait for Heroes 7. You won't be missing out on anything at all.